Process for producing thin metallic strip by continuous casting

ABSTRACT

A process for producing a thin metallic strip by continuous casting, comprising forming a pouring basin portion for a molten metal between a pair of rotary cooling drums respectively having shafts parallel to each other and a pair of side gates in contact with the end faces of the cooling drums and pouring molten metal into the pouring basin portion for molten metal to continuously cast a thin cast strip, characterized in that the casting is conducted while vibrating the side gates at a frequency, f (Hz), determined according to the following formula in a direction substantially horizontal to an imaginary line formed by connecting the shaft centers of the cooling drums to each other: 
     
         aA+b+cV≦f≦50                                 (1) 
    
     wherein 
     A is the numerical value of the amplitude (mm) of the side gate at the kissing point portion of the cooling drum and is in the range of from 0.5 to 5 mm; 
     V is the numerical value of the casting rate (m/min) previously determined from a desired casting sheet thickness; and 
     a=2, b=5, and c=0.1.

DESCRIPTION

Technical Field

The present invention relates to a process for producing a thin metallicstrip by continuous casting in a twin drum system and particularly to amethod of vibrating a side gate constituting a pouring basin portion.

Background Art

A conventional continuous casting process using a twin drum systemcomprises forming a pouring basin portion for a molten metal comprisinga pair of rotary cooling drums respectively having shafts parallel toeach other and a pair of side gates respectively in contact with the endface of the cooling drums, solidifying the molten metal poured into thepouring basin at a stage leading to the kissing point while cooling themolten metal by means of the cooling drums, thereby forming a thin caststrip, and pulling the cast strip downward.

In casting a thin strip by the above-described process, a gap oftenoccurs between the end face of the cooling drum and the side gate inpress contact with the end face of the cooling drum. In this case, themolten metal enters the gap, or solidified matter adheres to the surfaceof the side gates and grows, so that it frequently becomes difficult toconduct casting due to the breaking of a solidified shell or entrainmenton the cooling drum attributable to the formation of casting fins.

In order to solve this problem, Japanese Unexamined Patent Publication(Kokai) No. 60-166146 discloses a method of vibrating the side gates inthe horizontal direction.

Since, however, the object of the technique disclosed in theabove-described document is to remove solidified matter solidified andgrown on the surface of the side gates, the proper range ofreciprocating movement of the side gates in the horizontal direction isas follows.

1 cycle: 0.25 to 5.0 sec

degree of movement in each direction, from rest position: 5 to 20 mmSpecifically, the amplitude of the said gate vibration is in the rangeof from 10 to 40 mm, and the number of vibrations per se (frequency) isin the range of from 5 to 0.2 Hz. That is, the above-described techniqueis characterized in that the vibration is conducted slowly but with alarge amplitude.

The present inventors have conducted various studies on theabove-described technique and, as a result, have found that althoughsuch means is effective in preventing the formation of casting fin, adelay of solidification of the molten metal occurs at the end portion ofthe cooling drum to form a porosity and a large secondary casting fin.

Specifically, when the amplitude of the side gate becomes large, a shellwhich is growing on the cooling surface of the drum is unfavorablyfloated from the cooling drum by shearing stress, which delays thedevelopment of the shell, so that there occurs a delay insolidification.

An object of the present invention is to remove solidified matter formedon the side gates and to prevent the delay of solidification at the endportion of the cooling drum.

SUMMARY OF THE INVENTION

In order to attain the above-described object, the present invention hasthe following constitution. Specifically, the present invention isdirected to a process for producing a thin metallic strip by continuouscasting, comprising forming a pouring basin portion for a molten metalbetween a pair of rotary cooling drums respectively having shaftsparallel to each other and a pair of side gates in contact with the endface of said cooling drums and pouring said molten metal into saidpouring basin portion for a molten metal to continuously cast a thinstrip, characterized in that a side gate frequency, f (Hz), isdetermined according to the following formula (1) using the initialnumerical value (in the range of from 0.5 to 5 mm) of a side gateamplitude, A, at a kissing point of said pouring basin portion for amolten metal and the numerical value of the casting rate, V (m/min),previously determined from a target sheet thickness of a cast strip andcasting is conducted while vibrating said side gates at the determinedfrequency, f, at an amplitude, A:

    aA+b+cV≦f≦50                                 (1)

The range of the side gate amplitude, A, is determined so that theoccurrence of casting fin and the delay of solidification can beavoided. The casting rate, V, is limited, as one of the castingconditions, to a particular range for each apparatus and is continuouslymeasured by means of a casting rate detector provided on, for example, ashaft of the drum, and when the casting rate is varied, at least one ofthe amplitude and the frequency of the side gate vibration is adjustedaccording to the formula (1) .

In the above-described formula (1), a=2, b=5, and c=0.1.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a diagram showing the relationship between the amplitude ofside gate vibration, the frequency of side gate vibration and the delayof solidification when the casting rate is 40 m/min;

FIG. 2 is a diagram showing the relationship between the amplitude ofside gate vibration, the frequency of side gate vibration and the delayof solidification when the casting rate is 80 m/min;

FIG. 3 is a diagram showing the relationship between the amplitude ofside gate vibration, the frequency of side gate vibration and the delayof solidification when the casting rate is 120 m/min;

FIG. 4 is a perspective diagram showing the state of practice of thepresent invention;

FIG. 5 is a partly broken side view of the principal portion of avibrating device for side gates;

FIG. 6 is a cross-sectional view taken on line I--I of FIG. 5.

BEST MODE FOR CARRYING OUT THE INVENTION

The best mode for carrying out the invention will now be described indetail.

At the outset, an embodiment of the present invention will be describedwith reference to FIG. 4. In the present invention, use is made of acasting apparatus shown in FIG. 4, that is, a casting apparatus whereincooling drums 1, 1 provided with a cooling mechanism in the insidethereof are provided in such a manner that the shafts are parallel toeach other and a pair of side gates 2, 2 are provided in contact withthe end face of said cooling drums 1, 1, thereby forming a pouring basinportion 4. A molten steel 5 is poured into the pouring basin portion 4through a molten metal pouring nozzle 3, and the cooling drums 1, 1 arerotated in the direction of arrows a, a to cool and solidify the moltenmetal 5. The solidified layer is subjected to press contact at thekissing point 6 to form a thin cast strip 7.

In the above-described apparatus, a laboratory apparatus used to obtainan effect of the present invention is shown in FIGS. 5 and 6. In thedrawings, side gate 2 is pressed against a pair of cooling drums 1 bymeans of a pressing device 15 through a vibrating plate 8 to form apouring basin portion. The cooling drums 1 are rotated while conductingslide contact with a refractory material 2-1 provided on the surface ofthe side gate 2. On the back face of the vibrating plate 8 for fixingthe side gate 2, a bearing 3 is provided below the molten metal surface5-1 and above the kissing point 6 of the cooling drum, preferably aroundthe center of gravity of the side gate or the center of gravity of asurface in contact with the molten metal of the side gate, and the tipof a vibration supporting shaft 12 fixed to a frame 14 is inserted androtatably mounted in the bearing 13. On the other hand, a guide 11 isprovided below the kissing point 6 of the cooling drum, and a slider 10is slidably fit into the guide 11. The eccentric tip of an excitationshaft 9 which is removably and rotatably supported on the frame 14 issupported on the slider 10 so that it can be rotated by the shaft. Inthis state, when the excitation shaft 9 is rotated by means of a drive(not shown), the slider 10 reciprocates by sliding within the guide 11.This causes the vibrating plate 8 to be moved about the vibrationsupporting shaft 12, thereby vibrating the side gate 2 fixed to thevibrating plate 8. Thus, vibration is imparted to the side gate 2 in adirection horizontal to an imaginary line formed by connecting the shaftcenters of the cooling drums to each other.

The present inventors have produced a thin strip by using the abovelaborator apparatus shown in FIG. 5 according to a process whichcomprises pouring a molten SUS304 austenite-based stainless steel intothe pouring basin portion and subjecting the molten steel to continuouscasting at a casting rate, V, of 40 m/min to produce a thin strip. Inthis case, the amplitude, A (mm), and the frequency, f (Hz), were variedto evaluate the delay of solidification at the end portion of a caststrip. The results are shown in FIG. 1. The delay of solidification wasexpressed in terms of the length of delay of solidification in the widthdirection of a cast strip at the end portion of the cooling drum.

As shown in the drawing, when the side gate amplitude, A, is less than0.5 mm, it becomes difficult to peel off the solidified matter formed onthe wall surface of the fixed gate, so that the occurrence of castingfin became significant. On the other hand, when the side gate amplitudeexceeds 5 mm, a shear stress occurs between the cooling drum and theshell formed through the contact of the molten metal with the coolingdrum, so that the shell is pulled from the cooling drum by the shearstress. This causes a delay in solidification and in turn the occurrenceof porosity and a large secondary casting fin. On the other hand, whenthe side gate frequency, f, is less than a value determined by thefollowing equation

    f=a×A+b+c×V=2×A+5+0.1×40=2A+9      (Hz)

wherein a=2, b=5 and c=0.1,

it becomes difficult to peel off solidified matter formed on the wallsurface of fixed gates, so that the occurrence of casting fin becomessignificant, which causes the delay of solidification to becomesignificant. When the side gate frequency, f, exceeds 50 Hz, thereoccurs breaking of side gate refractories which is causes operationfailure.

Thus, it was found that in the above-described case, good results can beobtained by vibrating the side gates at an amplitude, A, in the range offrom 0.5 to 5 mm and a frequency, f, in the range of from (2A+9) to 50Hz. This frequency range suggests that when an increase in theamplitude, A, is intended, it is necessary to increase the frequency forthe purpose of preventing peeling of the shell.

Then, a steel of the same type as that used above was cast at a castingrate, V, of 80 m/min. As shown in FIG. 2, the lower limit of the sidegate frequencies corresponding to each side gate amplitudes wasincreased, so that the proper range became narrow. When the castingrate, v, was 120 m/min, as shown in FIG. 3, the lower limit of the sidegate frequency was increased. Thus, when the casting rate, V, isincreased, if the frequency is around the lower limit value, thefrequency should be increased to a suitable frequency.

Specifically, the present invention is characterized in that thefrequency of the side gate vibration, and the amplitude, at the kissingpoint portion are properly selected according to the casting rate. Thevibration of the side gate under such a condition shortens the delay insolidification in the direction of width at the end portion of thecooling drum, which reduces the amount of trimming at the time of coldrolling, which contributes to a remarkable improvement in productionyield.

The casting rate, V, is previously determined for each casting machineby determining the thickness of a sheet to be cast according to thefollowing equation. ##EQU1##

When the arc angle and the drum diameter are 40° and 1200 mm,respectively, ##EQU2## wherein K represents the coefficient ofsolidification, t represents the contact time and B represents aspecified sheet thickness. The V value can be determined because K is avalue inherent in the casting machine and B is known before casting.

Therefore, the side gate amplitude and the initial value of thefrequency are determined based on the casting rate, V.

Although the present invention has been described based on SUS304austenite stainless steel, it was confirmed through various tests thatthe vibration of the side gates according to the above-describedequation and the above-described numerical values is very effective insuppressing the occurrence of casting fin and preventing the delay ofsolidification when the steel is an austenite stainless steel. Further,in other types of steel as well, the application of vibration to theside gates in substantially the same manner as that described above iseffective.

EXAMPLES

Steels listed in Table 1 were cast at three casting rates, that is, 40m/min, 80 m/min and 120 m/min into thin cast strips having thicknessesgiven in Table 2. The side gate vibration conditions, yields, etc. inthis case are given in Table 2.

In Comparative Examples Nos. 2 and 5, the frequency relative to theamplitude was low and outside the scope of the present invention, sothat the delay of the solidification was large and the yield was reduceddue to an increase in the degree of trimming.

Regarding the steels used, A represents a SUS304 austenite-basedstainless steel, B represents a low-carbon Al killed steel, C representsa silicon steel sheet, and D represents a ferrite-based stainless steel.

                                      TABLE 1                                     __________________________________________________________________________    No.                                                                              C  Si Mn  P  S  Ni Cr  Al O   N   Nb                                       __________________________________________________________________________    A  0.045                                                                            0.45                                                                             1.01                                                                              0.030                                                                            0.005                                                                            8.30                                                                             18.20                                                                             0.002                                                                            0.0058                                                                            0.0325                                                                            0.020                                    B  0.035                                                                            0.05                                                                             0.20                                                                              0.003                                                                            0.001                                                                            0.01                                                                              0.005                                                                            0.030                                                                            0.0025                                                                            0.0034                                                                            0.003                                    C  0.005                                                                            3.15                                                                              0.005                                                                            0.003                                                                            0.003                                                                            0.02                                                                              0.09                                                                             0.04                                                                             0.0011                                                                            0.0020                                                                            0.001                                    C  0.060                                                                            0.30                                                                             0.21                                                                              0.019                                                                            0.001                                                                            0.12                                                                             16.50                                                                             0.034                                                                            0.0034                                                                            0.0175                                                                            0.002                                    __________________________________________________________________________

                                      TABLE 2                                     __________________________________________________________________________           Sheet           Length of                                                                           Degree of                                        Casting                                                                              thick-                                                                            Side gate conditions                                                                      delay of                                                                            trimming                                            rate,                                                                             ness,                                                                             amplitude,                                                                          frequency,                                                                          solidifica-                                                                         on one                                                                              Yield                                                                             Classifi-                                                                            Steel                           No.                                                                              m/min                                                                             mm  mm    Hz    tion, mm                                                                            side, mm                                                                            %   cation used                            __________________________________________________________________________    1  40  3.8 1     10    0.8   0     100 Ex.    A                               2  40  3.8 1      5    30.0  35    91  Comp. Ex.                                                                            A                               3  40  3.8   0.5 20    1.0   0     100 Ex.    A                               4  40  3.8 5     20    1.5   0     100 "      A                               5  40  3.8 3     10    25.0  20    95  Comp. Ex.                                                                            A                               6  80  2.3 3     15    5.0   7     98  Ex.    A                               7  80  2.3 3     50    0.5   0     100 "      A                               8  120 1.8 3     50    1.0   0     100 "      A                               9  40  3.5 1     15    3.4   5     98  "      B                               10 80  2.2 3     50    1.6   4     98  "      B                               11 40  3.6 1     15    2.4   4     98  "      C                               12 80  2.4 3     50    1.8   3     99  "      C                               13 40  3.8 1     15    1.7   3     99  "      D                               14 80  2.3 3     50    0.5   0     100 "      D                               __________________________________________________________________________

INDUSTRIAL APPLICABILITY

As described above, according to the present invention, since no delayof solidification at the end portion of the cast strip occurs, it isunnecessary to conduct trimming, which contributes to a remarkableimprovement in the yield, so that the effect of the present invention,on the production of a cast strips of stainless steels and other steelsis very large.

We claim:
 1. A process for producing a thin metallic strip by continuouscasting, comprising forming a pouring basin portion for a molten metalbetween a pair of rotary cooling drums respectively having shaftsparallel to each other and a pair of side gates in contact with the endfaces of said cooling drums and pouring said molten metal into saidpouring basin portion for a molten metal to continuously cast a thincast strip characterized in that said casting is conducted whilevibrating said side gates at a frequency, f (Hz), determined accordingto the following formula in a direction substantially horizontal to animaginary line formed by connecting the shaft centers of said coolingdrums to each other:

    aA+b+cV≦f≦50                                 (1)

wherein A is the numerical value of the amplitude (mm) of the side gateat the kissing point portion of the cooling drum and is in the range offrom 0.5 to 5 mm; V is the numerical value of the casting rate (m/min)previously determined from a desired casting sheet thickness; and a=2,b=5, and c=0.1.
 2. The process according to claim 1, wherein the castingrate is detected during casting and at least one of the frequency, f,and the amplitude, A, is adjusted according to the formula (1).